This invention relates to the art of repairing damaged tubes constituting part of a tube assembly. It has particular relationship to the repair, and specifically the sleeving, of the damaged tubes which conduct the coolant of a steam generator of a nuclear-reactor plant. The damage to the tubes is predominantly corrosion. This invention has unique utility in the repair of the damaged or corroded tubes of steam generators of such plants. It may have more general use, and to the extent that its principles are used more generally, such use in regarded within the scope of equivalents of this application or of any patent which may issue on or as a result thereof.
Access to repair damaged tubes is obtained through manways in the channel head of a generator. A tube sheet extends across the channel head and the tubes are welded into the tube sheet and open into the channel head. The access region under the channel head is radioactive and it is essential that the presence of personnel in this region be minimized. In addition, the channel head is composed of metal. To preclude hazard to personnel, it is necessary that no conductors carrying power at commercial voltages (110 volts) or higher be present within the channel head. Conductors operating at about 24 volts or the like may be present.
The sleeving of the tubes involves several processes. In the past, plugs were inserted in the corroded tubes of a steam generator to prevent the flow of radioactive coolant into the steam or feedwater. The plugging of a tube reduces the efficiency of the generator by a magnitude corresponding to the relative quantity of coolant carried by the plugged tube. Substantial loss of efficiency, in some cases of the order of 15%, has been experienced in nuclear-reactor plants in which a large number of the tubes were plugged. To overcome this economic loss it has become necessary to drill out the plugs and sleeve the tubes. A tool to drill out plugged tubes is provided for this purpose. A tool is also provided for inserting the sleeve blanks into damaged tubes, those that are drilled out as well as others. Once a tube is sleeved, it is desirable to secure the sleeve in the tube and to seal the region between the sleeve and tube against penetration of coolant or feedwater. This object is accomplished first by expanding the sleeve at its internal or upper end and in the portion of the sleeve in the tube sheet, i.e., at the lower end of the sleeve, and then by hard rolling the expanded sleeve at its rim and within the tube sheet. Expansion of the sleeve is disclosed in U.S. Pat. No. 4,513,502 granted Apr. 30, 1985 to John P. Vogeleer for "Measuring of Tube Expansion" and assigned to Westinghouse Electric Corporation. The upper and lower hard rolling are carried out by tools each including a roller which is inserted in the sleeve after the sleeve has been inserted in the damaged tube. The tool for upper hard rolling is sometimes referred to as the high hard roller and the tool for the lower hard rolling is referred to as a low hard roller. Each tool is provided with locking pins by means of which it is firmly suspended from the tube sheet. The locking pins engage tubes in the sheet which are adjacent to a tube being sleeved.
The drill, the sleeve-insertion tool, and the upper and lower hard rollers are driven by hydraulic motors mounted on each tool. The locking pins are driven and secured in the adjacent tubes hydraulically. The hazards of high voltage in the channel head are thus overcome. While hydraulic drives are preferred, occasions may arise where some or all of the above-described functions may be performed by pneumatic drives and the practice of this invention with pneumatic drives is contemplated as within its scope. Secondary operation, such as the raising of the high hard roller to the rolling position, are carried out by low voltage, usually 24 volt, to 28 volt motors.
It is necessary that a hydraulic or pneumatic servo control system be provided which shall control the above-described tools in a manner consistent with, and with the precision demanded by, the process requirement. Control systems in accordance with the prior art teachings which are available are single-purpose systems which are not suitable for use in the environment of the reactor plant and of the steam generator. It is an object of this invention to overcome the drawbacks of the prior art and to provide integrated apparatus for performing the above-described sleeving processes with the precision and under conditions required in a nuclear-plant containment environment.